Unmanned aircraft often use a terrain contour matching (TERCOM) navigation system that compares altitude samples with an on-board contour map of the terrain to determine the aircrafts position. A typical TERCOM navigation system first collects a sequence of altitude samples measured by devices such as a radar altimeter or laser altimeter. The altitude samples are then assembled into a two-dimensional array that associates a horizontal position (e.g. a longitude and latitude) with each of the altitude samples. The TERCOM navigation system also includes a contour map of a region of terrain that associates altitudes with two-dimensional horizontal positions. The TERCOM navigation system establishes a first reference point that represents the aircraft's position based on the two-dimensional array, while establishing a second reference point that represents the aircraft's position based on the contour map. The TERCOM navigation system then correlates a selected area from the contour map (hereinafter referred to as the selected reference basket) across the entire sequence of altitude samples to achieve a best fit solution. The aircraft's position error is then determined based on any resulting delta between the first and second reference points. If there is no position error, then the first and second reference points will correspond to the same point from the best fit solution. If the aircraft is off in one or both of the latitude direction or the longitude direction, there will be a corresponding offset between the reference points.
As previously mentioned, the traditional TERCOM navigation system correlates the reference map across the entire sequence of altitude samples to achieve a best fit solution. This traditional TERCOM technique results in two problems. First, to calculate the position error using a correlation algorithm, the TERCOM navigation system must batch process the entire sequence of altitude samples at once. This batch processing of the sample points results in significant processor loading while the algorithm is executed, and further results in a delay (2-3 seconds, for example) before a position error estimate is available to the aircraft for making any required course correction. Second, the portion of the contour map used to perform the correlation must be pre-selected based on a pre-flight plan. If the reference map is too large, the correlation algorithm will waste time and processor resources needlessly calculating a best fit solution for areas of the terrain that are not relevant. If the reference map is too small, the correlation algorithm may not converge on a best fit solution. Therefore, the size of the reference map is largely based on the expected position error. The greater the expected error, the larger the correlation area provided by the reference map needs to be. The less the expected error, the smaller the correlation area provided by the reference map needs to be. One additional factor that must also be considered in selecting the size of reference map is the expected terrain slope variation. The terrain slope variation dictates how much information is provided by an area of terrain. For example a terrain comprising mostly desert is relatively smooth and provides limited variations in terrain slope. Thus one area of the desert terrain's surface provides little information to distinguish it from another. In contrast, a mountainous terrain provides many variations in terrain slope. One area of the mountainous terrain's surface is less likely to be duplicated by another area and thus provides more information than a similarly sized area of desert terrain. Accordingly, a reference map covering a desert terrain must be relatively larger than a reference map covering a mountainous terrain in order for the correlation algorithm to be sure to converge on a best fit solution. For this reason, these traditional TERCOM navigation systems must be pre-programmed with a predetermined flight plan in order to know the type of terrain the aircraft will fly over and thus select the correct size reference map for correlating.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for improved systems and methods for TERCOM navigation.